Final answer:
The errors in the sentences have been identified and corrected.
Step-by-step explanation:
Sentence 1:
The error in this sentence is the use of the present continuous tense 'are having' instead of the simple present tense 'have'. The correct sentence is 'We usually have lunch out on Sundays.'
Sentence 2:
The error in this sentence is the unnecessary use of the word 'be' in the phrase 'what will we be doing'. The correct sentence is 'We can take a picnic but what will we do if it starts to rain?'.
Sentence 3:
The error in this sentence is the use of the present continuous tense 'was having' instead of the simple past tense 'had'. The correct sentence is 'She stayed at home because she had a cold.'